After trying to implement a minor controlled burn to tame the wild forests surrounding my home, I managed to create what I shall heretofore refer to as The Burning Land.

It occurred to me that we do not properly understand fire and its ability to spread from treetop to treetop. I am quite certain that I witnessed fire jumping from leaf block to leaf block over a span of at least two blocks of air, so clearly trees can catch fire even if they are not adjacent to a blaze.

So what is the minimum safe spacing between trees?

(A similar question was raised in Properties of Fire?, but I thought this question could be focused on forest fires, which the other question doesn't appear to address in sufficient detail.)

So I did some tests and it appears as though three spaces is the absolute minimum, meaning fires can jump across two air blocks to ignite other flammable materials. However as you can see in the attached image, grass will also sometimes catch fire. So, to be safe when doing controlled burns, I definitely recommend a four-space buffer. If grass between your trees catches, that turns a safe three-space gap into an inferno friendly two-space gap.

I noticed at one point that if you dig straight down and plant a seed in dirt, the trunk will reach the surface before sprouting.

I had this great idea for a log factory. I dug all the way to the bottom, planted trees and waited (Actually it was multi-player, so I bailed and came back later).

Every time the trees had grown, instead of a tall, proud 64+brick long trunk I found a few burning trunk pieces (although there weren't many, there were enough to know that my experiment did work--they went from the bedrock to the surface).

At first I thought someone was setting the forest alight while I wasn't there, but later I figured out that fire from lava must be jumping THROUGH the solid stone around where I planted.

I don't know quite where the lava is, but it's common enough down at that level that you must almost always be close enough to catch fire.

Disappointing but I'm going to keep working on it.

Oh, ob. answer: If anyone has time to watch the process, my guess is that the occasional sparks sent off from fire and lava can set things within a few squares alight if they happen to hit--and that (possibly a bug) those sparks aren't stopped by solid objects.

I've been set alight before by being diagonally adjacent to a lava block.
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Jonathan DrainNov 7 '10 at 2:20

1

This is due to the North-East glitch, where entities can interact with water or lava diagonally if they are meeting at the north-east corner. It's due to a programming bug where the physics calculate the collisions 1 pixel off (or something like that). This is how people build water elevators.
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Kevin YJan 30 '11 at 20:42